gnrc_lorawan: fix gnrc_pktbuf_release_error (introduced by #16080)

[jia200x]

Contribution description

This PR fixes an issue introduced by #16080, in which the wrong pktsnip is released on TX errors.
I removed the head variable and asserted "payload" just to avoid this kind of mistakes in the future.

Testing procedure

It's quite hard to test LoRAWAN right now if we don't have working setups with TTN, but I have a running instance of Chirpstack. I will post results ASAP.

If the transmissions fails (e.g the node didn't join), there's an assertion on master:

2021-07-06 13:59:26,331 # txtsnd 3 02 test
2021-07-06 13:59:26,338 # sys/net/gnrc/pktbuf/gnrc_pktbuf.c:94 => 0x8000165
2021-07-06 13:59:26,340 # *** RIOT kernel panic:
2021-07-06 13:59:26,341 # FAILED ASSERTION.

With this PR:

2021-07-06 14:28:20,569 # reboot
2021-07-06 14:28:20,613 # NETOPT_RX_END_IRQ not implemented by driver
2021-07-06 14:28:20,621 # main(): This is RIOT! (Version: 2021.07-devel-489-g13274-pr/gnrc_lorawan_fix_release)
2021-07-06 14:28:20,626 # Initialization successful - starting the shell now
> txtsnd 3 01 test
2021-07-06 14:28:25,992 # txtsnd 3 01 test
> txtsnd 3 01 test
2021-07-06 14:28:26,840 # txtsnd 3 01 test
2021-07-06 14:28:27,021 # txtsnd 3 01 test
2021-07-06 14:28:27,169 # txtsnd 3 01 test
2021-07-06 14:28:27,318 # txtsnd 3 01 test
2021-07-06 14:28:27,460 # txtsnd 3 01 test
2021-07-06 14:28:27,603 # txtsnd 3 01 test
2021-07-06 14:28:27,756 # txtsnd 3 01 test
2021-07-06 14:28:29,070 # txtsnd 3 01 test
> txtsnd 3 01 test
2021-07-06 14:28:30,244 # txtsnd 3 01 test

Issues/PRs references

#16080

[fjmolinas]

ACK, as @jia200x mentions its hard to test with no gateway, I don't have access to one ATM, so trusting his testing, and the change is sensible.

[jia200x]

thanks for the review!